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(no email)
Date: Fri Aug 08 2003 - 22:35:36 EDT
>
> Question 1 -- it looks like the forward voltage as specified from the
"pdf"
> spec sheet shows the forward voltage is 3.6 - 4.0
>
> Does this mean that all you have to do is connect maybe 3 or 4 of these
LEDs
> in series for a 12 volt system?
> 4 LEDs in series would put 3 volts across each LED.
>
REPLY:
NO that won't work. The voltage across the LEDs is constant. If you try to
put more voltage across them than they naturally take you blow the snot out
of them. To limit the current in the simplest manner you use a resistor.
>From a 12 volt supply, subtract the 4 volts across the LED leaving 8 volts
for the resistor. To get 20 milliamps the resistor would be (12-4)/.02 =
400 ohms but it should be a little higher because when your batteries reach
14.2 on charge the current would rise to (14.2-4)/400=25 milliamps.
So from 12 to 14.2 volts you get a 25% change in current or a 25% change in
brightness.
Now look at what happens with two LEDs in series. Your reisitor for 20
milliamps at 12 volts is (12-8)/.02=200 ohms
But when you get to 14.2 volts you now have (14.2-8)/200=50 milliamps, a
250% change in current and brightness instead of a 25% change.
If you put three in series, Your resistor for 20 milliamps at 12 volts is
(12-12)/.02=zero ohms. The moment the voltage rises above 12 the current
will be high enough to blow them up.
> Question 2 -- Appreciate learning how to connect this voltage regulator to
3
> or 4 lights in series?
REPLY:
With a 12 volt supply and a fairly good quality solid state constant current
regulator the maximum number you can put in series is two because you have
to have enough voltage across the regulator (like the resistor above) so
that it can control the current.
>
> Question 3 -- The LED is rated at 3000 mcd -- does this equate to wattage
or
> lumens for the amount of light the LED puts out?
REPLY:
mcd stands for millicandles or thousandths of the output of the standard
candle. What is the standard candle? I'm glad you asked. It is the light
put out by the standard candle (when lit) that is kept in the vault at the
tower of London, at zero degrees C and constant pressure. Seriously though,
the light is minimal even at 3 times a candle. You don't want a "wide
viewing angle" because that will disperse the light before it gets anywhere.
Wide angle LEDs are only used for indicators on a panel that have to be
viewed by an opertor from any angle. For useable light output you want the
narrowest beam you can get. Anything else will be useless. Having narrowed
the beam to get usefull brightness, you will need more and more LEDs beside
each other if you want a wider beam.
> Thanks,
>
> Al
> Pacific Trawler 37
> Palm Coast, Florida
Also keep in mind that you should have voltage breakdown protection on the
circuit otherwise it will blow up every time a lightning strike comes within
100 yards. Also keep in mind that good design would include a reverse
voltage protection circuit so that if some idiot connectes it up backwards
you don't blow up the LEDs. Their reverese voltage characteristics are very
poor.
More information than you wanted huh?
Andina Foster,
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